Universal Mobile Telecommunication System (UMTS), Universal Terrestrial Radio Access Network (UTRAN), a Long Term Evolution (LTE) network called Evolved UTRAN (E-UTRAN), and an LTE advanced network are some examples of RANs. A base station, or an evolved Node B (eNB) in the 3rd Generation Partnership Project (3GPP) LTE, assigns radio resources to a user equipment (UE) and signals this information to the UE. The UE in the present application is a mobile terminal or another type of device.
Transmissions are divided in LTE into 1 ms time periods called subframes. The radio resources, which are assigned to the UE, are available in a subframe on a certain channel of the LTE network, far example, on the Physical Downlink Shared Channel (PDSCH) or the Physical Uplink Shared Channel (PUSCH). Control channels are used for requesting and allocating radio resources, for example the Physical Downlink Control Channel (PDCCH) and Physical Uplink Control Channel (PUCCH) are the control channels of LTE. The eNB sends a downlink (DL) assignment or an uplink (UL) grant to the UE when the resources are assigned for the UE. Then the DE uses the resources for data reception according to the DL assignment, or for data transmission according to the UL grant. A transmission of a packet may fail. When a retransmission of the packet is needed, the retransmission is performed, for example, using Hybrid Adaptive Repeat and Request (HARQ).
HSPA (High Speed Downlink Access) was included in the 3GPP Releases 5 and 6 for downlink and uplink transmissions. “HSPA evolution”, also termed “HSPA-+”, is a work that has progressed in parallel to Long Term Evolution (LTE) work in the 3GPP. Many of the technical solutions in the HSPA evolution and LTE are quite similar. The HSPA evolution and LTE include technical solutions for mobile networks, home base stations (femto cells), and self-optimized networks. The HSPA evolution aims to improve performance for the end user by lower latency, lower power consumption, and higher data rates.
HSPA is based on Wideband Code Division Multiple Access (WCDMA). WCDMA utilizes the Time Division Duplex (TDD) or the Frequency Division Duplex (FDD) mode in order to provide total duplex communication on 10 ms frames. In TDD mode both the uplink data and the downlink data can be transferred over a single 5 MHz channel along with time multiplexing. Conversely, the FDD mode utilizes two separate uplink and downlink channels which are in one option separated by a 190 MHz band. In 1999 the 3GPP published such a version of WCDMA that provided data rates up to 2 Mbps. Later, the 3GPP defined WCDMA/HSPA that uses 16 QAM (Quadrature Amplitude Modulation) as the digital modulation scheme increasing the data rates up to 14.4 Mbps.
In wireless telecommunications, single-path propagation and multipath propagation are the phenomenons relating to reception of a radio signal. A single-path signal reaches a receiving antenna via a single path and is multipath signal reaches the antenna via two or more paths. Causes of the multipath propagation include atmospheric ducting, ionospheric reflection and refraction, and reflection from water bodies and terrestrial objects, such as hills and buildings. The to multipath propagation results in constructive and destructive interference, and phase shifting of the signal. The destructive interference causes fading. Also a single-path signal may fade. The fading needs to be modelled in order to demodulate the single-path signal or the multipath signal in a correct way, i.e. so that the original information can be extracted from it.
In the multipath propagation a signalling, which is traveled via some path to the receiving antenna, is termed a “multipath component”. Rician fading is an appropriate fading model when one multipath component dominates. Usually this dominating multipath component is received via a line of sight path. For example, Rayleigh fading is a fading model that provides better reception quality than Rician fading, if there is no dominating multipath component.
A term “receiver” refers in the present application to an entity that is used in a reception of a signal. A rake receiver is typical example of a receiver.
FIG. 1 shows a radio modem 101 equipped with a reception antenna 102 and a rake receiver 103. The radio modem 101 counts the effects of multipath fading by using several “sub-receivers” called fingers 104-106. Each finger decodes one multipath component 107-109 that has traveled via a different path from the transmitting antenna 110 to the receiving antenna 102. In this example, the multipath component 107 has traveled via the line of sight path to the antenna 102 and the other multipath components have reflected once from either of hills 111 before they have reached the antenna 102. The line of sight path is the shortest path to the antenna 102 and the other paths are longer. The longer the path the longer delays are included in the multipath component. The delays are estimated for each finger by using the following method. First, the radio modem 101 identifies the points of times at which significant energy packs are detected in the signal received through the antenna 102. The time differences between the multipath components 107-109 can be calculated on the basis of the energy peaks measured from the multipath components. The multipath component 107 is mapped to time difference t1 and the value of t1 is 0 because the multipath component 107 has traveled via the shortest path to the antenna 102. The other the multipath components 108 and 109 have traveled longer in meters and in milliseconds. The multipath component 108 is mapped to time difference t2 and the multipath component 107 is mapped to time difference t3. It is obvious on the basis of the path lengths shown in the FIG. 3 that t1<t2<t3. The fingers 104-106 are then adjusted using correlators 112-114 with the time differences t1, t2, and t3. After the use of the correlators 112-114 a phase and amplitude alignment 115 is performed. Now the multipath components 107-109 are adjusted to be as good as possible and they are combined in a combiner 116. The combiner 116 extracts data from the combined signal and outputs the extracted data 117. A checking unit 118 checks whether the extracted data 117 corresponds to the original data which the transmitting antenna 110 signalled. The checking unit may do this, for example, by using Cyclic Redundancy Check (CRC). If the extracted data is erroneous on the basis of the CRC, the checking unit 118 sends through a transmitter 119 a retransmission request 120. After that the retransmission of the data is expected to happen through the transmitting antenna 110. Otherwise, if the extracted data 117 is correct, it is output from the radio modem 101.
As mentioned in the above, geographical formations, such as hills and valley, and infrastructure, such as buildings, may deteriorate signal quality. When a UE moves, the signal quality may deteriorate for many reasons. For example, high movement speed of the UE causes the known Doppler phenomenon that deteriorates the signal quality. A long distance between the UE and the base station deteriorates the signal quality, especially when the UE arrives close to the border of the serving cell. The UE may need a handover at the border of the serving cell and the signal quality usually deteriorates during the handover. Handovers also relate to a multi-cell reception. The multi-cell reception and other issues relevant for the present application are discussed in the 3GPP TS 36.302, section 6.2.4.
In FIG. 1 the multipath components are originated from a single antenna, i.e. the transmitting antenna 110. Generally speaking, there may be two or more antennas transmitting multipath components. In addition to the multi-cell reception, the term “dual-cell HSDPA” is used in the prior art. The dual-cell HSDPA can be utilized so that two carriers of 5 MHz are allocated for the same UE. Then the signal reception at the UE comprises simultaneous reception of two signals on different carriers. This feature of the dual-cell HSDPA makes possible to accelerate the data rate of the transmission to 28 Mbps or even to 42 Mbps. The feature is discussed in: E. Seidel, J. Afzal, and G. Liebl, “White Paper—Dual Cell HSDPA and its Future Evolution”, Nomor Research GmbH, Munich, Germany, January 2009.
A conventional rake receiver described in FIG. 1 operates quite badly in HSPA networks, thus scientists have developed a number of new receivers. Many of the receivers use equalizers. For example, the following two documents describe receivers which are based use of equalizers: 1) M. Park, W. Lee, M. Nguyen, and H. Soo Lee, “Practical Chip-Level Equalizers in HSDPA”, Journal of Computers, vol. 3, no. 4, April 2008; and 2) A. Ghosh and R. A. Kobylinski, “Advanced receiver architectures for HSDPA and their performance benefits”, Texas Wireless Symposium 2005.
FIG. 2 illustrates a prior art radio modem 201 in which a receiver 202 receives through an antenna set 203 a signal, or a multipath component of the signal, as an input 204. The antenna set 203 comprises one antenna or more antennas. The receiver 202 outputs a bit set 205 as a response to the input 204. The bit set 205 is an input for a checking unit 206. If the bit set 205 is correct, the checking unit 206 outputs it. Otherwise, the checking unit 206 generates a retransmission request 207. Details related to the checking unit 206 and the retransmission request 207 are omitted from FIG. 2 because they are quite irrelevant from point of view of the present invention. In addition to the rake receiver 103 shown in FIG. 1 and the two equalizer-based receivers mentioned in the above, there are many other receivers which can be utilized in signal reception in the radio modem 201.
Radio frequency (RF) circumstances change all the time, especially when a UE moves. One prior art problem is that whatever receiver is selected as the receiver 202 of the radio modem 201, the receiver has its own weaknesses, i.e. it operates badly when the signal to be received deteriorates in some specific way.